Accurate Ring Strain Energies of Unsaturated Three-Membered Heterocycles with One Group 13-16 Element.

High-quality ring strain energy (RSE) data for 1unsaturated (CH)X parent rings, where X is a group 13-16 element, are reported in addition to the 2-isomers of the pnictogenirene rings. RSE data are obtained from appropriate homosdesmotic reactions and calculated at the DLPNO-CCSD(T)/def2-TZVPP//B3LYP-D3/def2-TZVP(ecp) level. 1Tallirene and 1plumbirene have unique donor-acceptor structures between an acetylene π(CC) orbital and an empty p orbital of a metallylene subunit (a Dewar-Chatt-Duncanson description) and therefore cannot be described as proper rings but as pseudocyclic structures. Also, 1indirene and 1oxirene lack ring critical points and constitute borderline cases of pseudorings. 1-Unsaturated rings exhibit enhanced RSE compared to their saturated homologues. The mechanism of ring strain relaxation by increasing the s character in the lone pair (LP) of group 15-16 elements is remarkable and increases on descending the groups. Furthermore, RSE is affected by the aromatic character of group 13 rings and certain aromatic or antiaromatic character in group 14 or 15-16 rings, respectively, which tend to vanish on descending the group as shown by NICS(1) values. 2-Unsaturated rings were found only for group 15 elements (although only 2-azirine shows a proper cyclic structure) and displayed lower RSE (higher stability) than the corresponding 1-isomers.